Transcript for Brushing Up on Electoral College Math; Key to Winning White House

Every four years, we watch as the vote comes in and whoever wins the electoral map, the electoral college, wins the presidency. You need 270 electoral votes to win. And so tonight, "gma" anchor and "this week" host george stephanopoulos with the bottom line on the best path for both campaigns. George? Reporter: In these final hours, david, scheduling is strategy, and for president obama, it's all about his midwest firewall -- iowa, wisconsin and that big prize, ohio. He's hit those states every day since friday. They are going to be his last three stops on monday. And this electoral map shows why. If president obama can win these three on top of the solid blue states democrats have taken in the last several elections, he'll have the 271 electoral votes he needs to win, even if governor romney sweeps all the rest of the battleground states. Which is why you do see governor romney making a late play for

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